Buckets for steam turbines have for some time been the subject of substantial developmental work. It is highly desirable to optimize the performance of these buckets to reduce aerodynamic losses. Optimally, the bucket profile should be designed to match aerodynamically the flow of the nozzle to provide the desirable operating characteristics over a large operating range. Factors which affect the bucket profile design include the active length of the bucket, the pitch diameter and the operating speed in subsonic flows. Damping and bucket fatigue are factors which must be considered in the mechanical design of the bucket. The buckets must also be tuned to avoid coincidence between their natural frequencies and the flow stimuli. These mechanical and dynamic response properties of the buckets as well as others, such as thermodynamic properties or material selection all influence the optimum bucket profile. In brief, next-to-last stage steam turbine buckets require a precisely defined bucket profile for optimal aerodynamic performance with minimum losses over a wide operating range.
Appropriate bucket profile design is also important to provide axially convergent flow passages between adjacent buckets to achieve maximum aerodynamic efficiency. Bucket designs in the past have also included coupling of groups of buckets at their outer tips employing covers. These couplings are used in the present bucket to reduce bucket response to stimuli in the working fluid, which could cause uncontrolled vibration of the buckets, for example, at their natural frequencies. Vibration, of course, is to be minimized or eliminated to avoid fatigue, crack initiation and eventual structural failure and these continuous couplings, of course, affect the aerodynamic properties of the buckets. It is important also for the covers to provide a seal at the tips of the buckets to minimize aerodynamic loss resulting from flow passing around the bucket tips.